Data storage mediums for optically recording data have received attention as mediums for recording a large amount of digital data.
A phase-change optical disc is one of recordable data recording mediums. The phase-change optical disc has a recording film melted by heating. By irradiating a rotating disc with a light beam of a semiconductor laser modulated based on data to be recorded, a phase change occurs on a part irradiated with the light beam on the recording film and data is recorded.
In the case of an intensive light beam, the part irradiated with the light beam on the recording film is heated to a high temperature and is rapidly cooled thereafter. Thus, the part irradiated with the light beam on the recording film becomes amorphous. In the case of a relatively weak light beam, the part irradiated with the light beam is heated to a moderate high temperature and is gradually cooled thereafter. Thus, the part irradiated with the light beam is crystallized. Normally the part having become amorphous is referred to as a mark and the part crystallized between marks is referred to as a space. Binary data is recorded by using the mark and the space. The string of the marks and the spaces is formed on a track which is spirally provided on the optical disc. Normally a laser power of an intensive light beam is called peak power and a laser power of a weak light beam is called bias power.
When data recorded on the phase-change optical disc is read, a weak light beam not causing a phase change of the recording film is emitted to the optical disc and reflected light is detected. Normally the mark having become amorphous has a low reflectivity and the crystallized space has a high reflectivity. Thus, a difference in quantity of reflected light between the mark and the space is detected to generate a reproduction signal, the reproduction signal is binarized, and then demodulation is performed so as to acquire recorded data.
As a method for recording data on the phase-change optical disc, mark position recording and mark edge recording are available. Normally mark edge recording (mark length recording) can obtain a higher recording density of information. A longer mark can be recorded in mark edge recording as compared with mark position recording.
When a light beam at peak power is emitted to the phase-change optical disc to record a long mark, the rear of the mark has a larger width in the radius direction due to the heat accumulation of the recording film. Thus, there arises a problem that undeleted data remains during direct overwriting and signal crosstalk occurs between tracks, which results in the seriously degradation of the signal quality.
In order to solve the problem, for example, Japanese Patent Laid-Open No. 9-7176 discloses that a mark formed by the mark edge recording is divided into a leading edge, an intermediate portion, and a trailing edge, the leading edge and the trailing edge are each formed by a single laser pulse of a predetermined length, and the intermediate portion is formed by a plurality of laser pulses each having a predetermined period. According to the method, since the intermediate portion is formed by the plurality of laser pulses, it is possible to suppress heat accumulation and prevent an increase in mark width. On the other hand, since the leading edge and the trailing edge of the mark is formed by the laser pulse of a predetermined length, sufficient thermal energy is applied to the recording film. Hence, even in the case of direct overwriting, it is possible to reduce jitter on the edges of a formed mark.
FIGS. 1 and 2 show examples of the waveforms of laser pulses used for forming marks of various lengths according to the conventional art. For example, data to be recorded is recorded according to mark edge recording, which uses recording modulation codes converted according to Run Length Limited (2, 10) modulating scheme. In this case, the recording modulation codes are present with the shortest length 3T to the longest length 11T where T represents a reference period of the recording modulation code of a recording mark. The mark and space, on which recording is performed according to mark edge recording, have a continuous length expressed by a length of the recording modulation code.
When these marks are formed on the optical disc, as described above, a plurality of laser pulses are employed as shown in FIG. 2, in each of marks having respective lengths. FIG. 6 shows a recording pulse train which generates a laser pulse for forming a mark of 6T. In FIG. 1, a pulse 801 at the front is referred to as a first pulse and a pulse 804 at the backend is referred to as a last pulse. Further, a pulse 802 and a pulse 803 between the first pulse and the last pulse are referred to as a multi-pulse train constituted of pulses of a period T.
The multi-pulse train of the mark 6T includes two pulses and the multi-pulse train of mark 7T includes three pulses. Moreover, the multi-pulse train of mark 5T is actually constituted of a single pulse. The number of pulses is increased by one as the mark length is increased by T. Conversely one pulse is reduced as the mark length is reduced by T. Therefore, mark 4T is only constituted of a first pulse and a last pulse and has no multi-pulse train. Moreover, mark 3T is constituted of a single pulse. Normally the first pulse has a width of 0.25 to 1.5 T and the last pulse has a width of 0.25 to 1 T. A single pulse constituting the multi-pulse train has a width of 0.25 to 0.75 T.
In the waveform of a laser pulse shown in FIG. 2, although the width of the last pulse is different from that of the waveform of the laser pulse shown in FIG. 1, a relationship between a mark length and the number of multi-pulse trains forming an intermediate portion is the same as the laser pulse of FIG. 1.
When marks are formed according to the above-described method, marks of different lengths can be readily formed by changing the number of pulses in the intermediate portion. However, according to this conventional method, when a speed for recording data is increased, for example, when data is recorded on an optical disc at a high transfer rate, since the response speed of a laser diode is not ideally high, the rising edge and the falling edge of a pulse becomes dull in a luminous waveform. Thus, a predetermined quantity of heat cannot be applied to the recording film of the optical disc. Particularly since the multi-pulse train has a pulse width of about 0.25 to 0.75T, for example, it becomes difficult to generate a pulse of a sinusoidal wave in some rising times and falling times of a laser. Hence, a correct mark cannot be formed.